Messages in this thread | | | Date | Mon, 26 Nov 2007 18:38:17 -0800 | From | "H. Peter Anvin" <> | Subject | Re: [PATCHv4 5/6] Allow setting O_NONBLOCK flag for new sockets |
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Linus Torvalds wrote: > >> The 6-word limit is a red herring. There is at least two ways to deal with it >> (and this doesn't mean wiping the legacy stuff we already have): >> >> - Let each architecture pick a calling convention and redefine the >> architecture-independent bits to take an arbitrary number of arguments. This >> is a one-time panarchitectural change. > > Not applicable on x86-32. > > The six-word limit is effectively a hardware limit there. Once it goes > past that limit, one of the words needs to be a pointer to extended > information that is fundamentally slower to access. Happily, only very > rare system calls do that (and none of them are of the simple variety > where we see a few cycles easily). > > On other architectures, we could more easily just use more registers. But > x86-32 is still a big part (bulk) of what matters for most people. >
Well, x86-32 and x86-64 are surprisingly similar here, for very different reasons (x86-64 is because there are only seven clobbered registers that aren't destroyed by the syscall instruction itself.)
However, on both of these we could make the user-space side cheaper, by making sure that we don't have to do additional copies in user space. For both these architectures, anything more than 3 parameters (i386) or 6 parameters (x86-64) will be already in memory on the stack, so if we can use that image as-is then we at least save the intra-user-space copy that goes along with it.
x86-64 requires some minor thought, since the obvious way of doing it (using arg register 6 to push in a pointer) would end up with a discontiguous frame. One can do it with something like this, although it's not clear to me it is a win at all (the more obvious sequence using XCHG isn't usable since XCHG locks unconditionally):
pop %r10 # Return address push %r9 # Argument 6 movq %rsp, %r11 push %r10 movq %rcx, %r10 syscall cmpq $-4095, %rax jae ... pop %r10 pop %r9 push %r10 retq
The number of registers do vary, obviously, with s390 being the smallest number (5).
> Immediately when you do anything but registers, it is much *much* more > costly. The "get_user()" and "copy_from_user()" stuff is not exactly slow, > but it's quite noticeable overhead for simple system calls. It gets worse > if this all is described by some indirect table setup.
True, of course, although we're talking here about different ways to pull arguments out of userspace memory; *definitely* agreed with that we don't want to have any additional indirection necessary.
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